The unique chemistry and internal molecular association of asphalt are responsible for its mechanical properties and hence for its use as a binder in asphalt concrete pavement. Large tonnages of asphalt are used every year in constructing roads throughout the world; thus any improvement in its properties, no matter how small, translates into considerable cost savings. The present invention is a means and method of improving the rheological properties of crumb rubber modified asphalt which is later used in pavement.
Adding crumb rubber to hot asphalt results in improved properties such as a wider useful temperature range. However, reports of performance of prior art crumb rubber modified asphalt concrete vary considerably; some pavements ravel after only a limited amount of use, sometimes there is a decrease in the mechanical properties of the binder, and sometimes there are inconsistencies in the applicability of the asphalt concrete in the field. These variations result from differences in the types of asphalt; the source, composition, size, and process of making the crumb rubber particles; and in the process of incorporating the crumb rubber particles into the asphalt. One of the primary problems with prior art crumb rubber modified asphalt binder is that it has poor settling characteristics; that is, the crumb rubber particles tend to settle or to produce new molecular size particulates which tend to settle to the bottom of the tank before the asphalt is mixed with aggregate. Another problem is that the particles do not disperse evenly through the asphalt. This may contribute to the problems noted above, particularly the ravelling.
It has been found that the quality of the asphalt after the addition of the crumb rubber is a function of the amount of interaction between the asphalt and the crumb rubber particles, which in turn is a function of the number of carboxylic sites on the surfaces of the particles. When the particles have a large number of carboxylic sites, they mix more evenly due to interaction with asphalt functional groups and the final product is much smoother; the crumb rubber particles also stay in suspension to a much greater degree than before.